1. Field of the Invention
The invention relates mainly to a liquid crystal composition suitable for use in a super twisted nematic device and so forth, and a super twisted nematic device containing the composition.
2. Related Art
In a liquid crystal display device, a classification based on an operating mode for liquid crystals includes phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA) and polymer sustained alignment (PSA). A classification based on a driving mode in the device includes a passive matrix (PM) and an active matrix (AM). The PM is further classified into static, multiplex and so forth, and the AM is classified into a thin film transistor (TFT), a metal-insulator-metal (MIM) and so forth. The TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to the production process. A classification based on a light source includes a reflection type utilizing natural light, a transmission type utilizing a backlight and a semi-transmission type utilizing both natural light and a backlight.
These devices contain a liquid crystal composition having suitable characteristics. The liquid crystal composition has a nematic phase. General characteristics of the composition should be improved to give a STN device having good general characteristics. Table 1 below summarizes the relationship between the general characteristics of the two. The general characteristics of the composition will be further explained based on a commercially available STN device. The temperature range of a nematic phase relates to the temperature range in which the device can be used. A desirable maximum temperature of the nematic phase is approximately 70° C. or higher and a desirable minimum temperature of the nematic phase is approximately −20° C. or lower. The viscosity of the composition relates to the response time of the device. A short response time is desirable to display moving images on the device. Accordingly, a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable.
TABLE 1General Characteristics of Composition and STN DeviceNo.General Characteristics of CompositionGeneral Characteristics of STN Device1wide temperature range of a nematic phasewide usable temperature range2small viscosity1)short response time3suitable optical anisotropylarge contrast ratio4positively large dielectric anisotropylow threshold voltage and small electricpower consumption5large elastic constant ratio (K33/K11)steep voltage-transmission curve andlarge contrast ratio1)A composition can be injected into a liquid crystal cell in a shorter period of time.
The optical anisotropy of the composition relates to the contrast ratio of the device. The product (Δn×d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the device is designed so as to maximize the contrast ratio. In this case, the optical anisotropy of the composition is mainly in the range of approximately 0.10 to approximately 0.20. A large dielectric anisotropy in the composition contributes to a low threshold voltage and a small electric power consumption in the device. Accordingly, a large dielectric anisotropy is desirable. A large elastic constant ratio (K33/K11) of the composition contributes to a steep voltage-transmission curve and a large contrast ratio of the device. Accordingly, a large elastic constant ratio is desirable.
A composition having positive dielectric anisotropy is used for a STN device. Examples of the liquid crystal composition having positive dielectric anisotropy are disclosed in the following patent documents. An example of a liquid crystal composition including an optically active compound is disclosed in the following patent No. 3. However, the helical pitch was not sufficiently short.    No. 1: JP H02-067232 A (1990); No. 2: JP H05-229979 A (1993); and No. 3: JP H06-200251 A (1994).
It is especially desirable that a composition has a high maximum temperature of a nematic phase, a low minimum temperature of a nematic phase, a small viscosity, a suitable optical anisotropy, a large dielectric anisotropy and a large elastic constant ratio and that the threshold voltage has a small temperature dependence and the dielectric anisotropy is hardly effected by the frequency applied to the device in the range of a high temperature to a low temperature.